Embodiments of the invention relate to apparatus and methods of atomic force microscopy, and spectroscopy, a subset of techniques in scanning probe microscopy.
In conventional Scanning Probe Microscopy (“SPM”), a probe with a sharp tip is rastered over a sample while the topography and material properties of the sample are measured by tracking changes in the changes in the tip-sample interaction. Atomic Force Microscopy (“AFM”), a version of SPM, uses a laser-photodetector system to track changes in the deflection of a cantilever beam with the tip at the free-end to measure the tip-sample interaction. To achieve high signal-to-noise ratio, most popular SPM techniques excite either the probe or the sample (mechanically, electrically, magnetically, etc. or a combination) using a sinusoidal waveform of known amplitude and phase and a narrow-band lock-in amplifier isolates the probe response at a single frequency. The frequency components of the probe response outside the detection bandwidth of the lock-in amplifier are inaccessible and are automatically discarded. In real material applications, however, the probe response is often nonlinear in frequency due to complications such as the changing nature of the tip-sample contact, as well as mode couplings, and transients in the response. Thus, the probe response outside the excitation frequency often contains valuable detailed information of the probe-sample interaction. The lock-in amplifier cannot access these frequency components in SPM, as their location in frequency space is unknown a priori, and as such simply ignores them.
Conventional AFM is also used for spectroscopy to study the sample response as a function of one or more parameters. For each spatial location, the excitation waveform is modulated such that each spectroscopic parameter is varied over a desired range. The excitation waveform is applied to the probe or the sample. The probe response, measured by the lock-in amplifier, at the probe excitation frequency as a function of spectroscopic signal parameters constitutes the spectroscopy measurement. Again, useful probe response outside the probe excitation frequency band is permanently lost.